Traveling wave device



Feb. 16, 1960 J. M. osEPcHUK TRAVELING WAVE DEVICE Filed June 16, 1958 2,925,518 l TRAVELING WAVE DEVICE John M. Osepchuk, Waltham, `Mass., assigner to Raytheon Company, a corporation of Delaware This.invention relates to a novel slow wave propa- Agating structure `for use in a traveling wave oscillator and `Patent of Edward C. Dench and AlbertDQLa Rue, Serial No. 562,472,1,fi1ed1anuary 3l, .1956,` now United States Letters Patent `No. 2,888,649, issued May 26, 1959, assigned to the same assignee as the present application. Such backward wave oscillators include a non-reentriant'slow wave energy propagatingnstructure or delay i line, 'anelongated electrode or sole spacedv from and disposed substantially parallel to said structure, and `an jfelectro'n' gun mounted adjacent `one end-of said sole `for producing an electron beam which traverses the interaction space bounded by th`e slow wave structure and the sole. `Because `off the interaction between the electron beam andl` the high frequency fields of waveenergy propagating along ,the delay line, energy is generated within Vthe tubefandfrnay be removcd therefrom by output coupling Qmeans coupled tothe delay structure adjacent the ends thereof" opposite the electron gun. The delay line `ingsuch` tubes is of the `interdigital type, which consists of a first-assembly including a first set of fingers each having one extremity attached to a common base ar- `range`d substantially along the length of the delay line,

and a second assembly including a second set of fingers each of which hasfone extremity attached to another `common base usually arranged more orfless `parallel to the common base of the first assembly. The ngersof the first land second `sets areintermeshed and thefree ex- Unid :States Patent O `of a second set `of `alternate fingers of the delay line of `the backward wave oscillator of Figs. 1 and 2; and

Referring to the drawing` a backward wave oscillator ice . over the ends ofthe fingers of the conventional interdigital line in the manner just described, the width of the delay line, that is, the dimension of the` delay line parallel to the magnetic field in traveling wave tubes using said delay line, is` decreased. This permits of a smaller magnet gap andminimization of magnetic weight, for a given magnetic field strength. Moreover, the beam width can be reduced and the space charge density` increased so that lower starting currents of crossed-ledl type backward wave oscillators can be obtained, i

Other objects and advantages of this invention will be more readily apparent as the description thereof progresses, reference being had tothe accompanying drawings wherein:

`Fig. 1 is a central cross-sectional view of a backward wave oscillator according to the invention;

Fig. 2 is a sectional view, taken along line 2 2 of Fig. l, of the backward wave oscillatorof Fig. l;

Fig. 3 is a view showing the construction of the delay line of the backward wave oscillator of Figs. 1 and 2;

Fig. 4 isa sectional view, taken along the line 4-4 of Fig. `2, indicating the construction of one of the fingers of a first set of alternatel fingers of the delay line of the backward wave oscillator of Figs. l and 2;

Fig; 5 is a sectional View, taken along line 5-5 `of Fig.` 2, indicating the construction of one of the `fingers Fig. 6 is a Vview showing a modified construction of the delay line ofthe backward wave oscillator of Figs. 1 and 2 particularly` suitable for low frequency tubes.

10 is shown in Figs. l and 2 which comprises a slow wave energy propagating structureor delay line 12, a cylindrical 'electrode 15, otherwise referred toasa sole, concentric with the delay line 12 and normally maintained negative with respect thereto, a lead-in assembly 16, an electron gun assembly 20 including at least `a cathode "21 and` heater 22, a magnetic field-producing means 25 and an output coupling means 17. The circular delay line `12 includes several interdigital fingers on elements 31 and 32 attached at one end to a` cylindrical electrically,conductivering or base 36, said base forming afpart'of-anevacuated envelope for tube 10. `The rietremities of each finger of a given assembly approaches,

`butdoies` not contact, the `common base of the other assembly. The length of the fingers of such a `delay line bears-a direct relationshiplwith the operatingufrequency of the oscillator, and often ismade slightly less thanionequarter of a `wavelength long at thesmallest wavelength in,` the operating frequency band. This length, in a line such. as just described, is measured from the point of attachement to the `common base to the free extremity of the finger. i

In accordance with the invention, the fingers of the `delayline includefthree mutually perpendicular copolanar portions. Specifically, each finger includes a first portion extending substantially perpendicular to a single uniplanar commonbase, otherwise referred to as a back wall, and

having itsv extremity attached to said common base, a

`second portion` substantially perpendicular to` the first `portionand,consequently parallel to the common base, .and athirdportion perpendicular to the `second portion and?extending` toward, but not contacting, the common base, The space betweenthe extremity of the third por :,mainder, of thetube envelope is made` up `of two oppositely disposed `cover plates 38 and 39 hermetically sealed to base 36.` As shown most clearly in Figs. 2 to 5, the delay line 12 includes one set of interdigital elements represented `by the reference numeral 31 and other set Oielements represented by the reference numeral 32. Elements 31 areattachedat one end adjacent one edge of the base 36, while a corresponding end of each ofi. the elements`i32 is attached tobase 36 along the opposite edge thereof.

Each of` the `elements 31 are characterized by three successively mutuallyperpendicular portions 31a, 31b', and 31e. The free extremity of portion 31a is attached to base 36 along, one edge thereof, portion 31a extends substantially.perpendicular to base 36 an-d merges With a portion 31h 4disposed substantially perpendicular to portion 31a and essentially parallel to the base 36. Portion 31 b, in turn, merges` with a portion 31a` which extends substantially perpendicular to portion 31b and `base `36 and substantiallyfparallel to portion 31a. i The free extremityfof `portion31c is spaced from `base 36. Each of the elements 32 similarly, are characterized by three portions correspond to portions 312i, 3`1b, and 31e, re-

lspectively, of fingers 31. The distance between the extremity of portion 31C, or 32e, as the case maybe,

and the base 36 preferably is made of the order of magnitude of the distance between adjacent fingers ..31 and v32,. The delay line 12 may be considered as the equiva- "lent of afolded parallel wire line wherein the wave energy'travels along a J-shaped .path between two adjacent fingers, then between the free end .of the finger andthe base, between the opposed surfaces ofthe Vnext set `of adjacent fingers and then in the space between the free yend of finger 32 and the base, etc. "If the space between the free ends of Ifingers 31 and 32 .is kept sub lstantially equal toV that between an .adjacent pair of -`fingers 31, 32, assuming of course that a .uniplanar base 36 is employed, the energy path is of uniform configuration throughout Ythe length of the delay line and this is equivalent to'having a parallel wire line of uniform spacing throughout.

The total length L of each finger, that is, the combined length of portions 31a, 31'b, and 31e taken along the center line of the cross section, may be of the order of a quarter wavelength at the smallest wavelength in the 'operating frequency band, while, at the same time, the width I of the delay line may be considerably vless than the 'total length L of .the fingers. Consequently the distance between cover plates 38 and 39, and, hence, magnetic gap M (see Fig. l.) may be made relatively small.

At relatively low operating frequencies, the total length The variation of inter-finger spacing along radial length. This causes variation in the electrical characteristics of the network along the length thereof, and extraneous coupling .other than the desired inter-finger coupling may result. By making the fingers 31 and 32 of Wedge-shaped cross section, as shown in Fig. 6, the spacing between fingers may be maintained essentially constant, and, if the spacing between .the free ends of the fingers land the base is made equal to the inter-finger spacing, the characteristics of the delay line Amay be made quite uniform regardless of finger length L.

It is possible to avoid troublesome efiects of direct current and radio frequency fields near the corners of the ngers by making the inside width of the sole 1.5 (distance between flanges) equal to, or less than, the sum of the length l and the distance between adjacent fingers.

'.A good choice is to make the total width of the sole 15,

including flanges, equal to the length l.

In order to prevent excessive movement of the 1r and 1r/2 cut-off wavelengths in the device, the distance be- 'tween the portion 31a of finger 31 and the cover plate '38 (or the distance between portion 32a of finger 32 and cover plate 39) should not be made too small. It has been found that a good choice is to make this distance equal to the distance between adjacent fingers.

Furthermore, in order not to derange greatly the prop- `ex'ties of the folded interdigital delay line from those of `a linear interdigital line of the prior art, it is desirable that the shortest distance between the inner edges of portions 31a and 3h.` of fingers 31 (and between the inner edges of portions 32a and 32e of lingers 32) be greater than twice the distance between adjacent lingers.

The sole consists essentially of a cylindrical segment of an electrically conductive material, which includes a web portion 45, bounded by an arcuate section 46, whose periphery consists of a major surface 47 and `L of the fingers becomes correspondingly long and, there- -fore, rthe radial distance between the inner faces of the portions 31b and 32b of the respective fingers 31 and `32 increases. vthe radial length of the fingers obviously increases with 4 member 54 is inserted within insert 52 and is xedly secured thereto. Supporting member 54, in addition to providing support for thesole 15, forms a portion of lead-in assembly 16 and permits passage of external circuit-connecting leads in a manner to be described later.

Sole 15 contains a slot 55 to accommodate the electron gun assembly 20. The electron gun 20 includes a mounting plate 56 which is attached to the web portion 45 of the sole assembly by means of screws 57. The electron gun 2t) includes a cathode 21, heater, not shown, a grid electrode 23, which may be used for control of beam current (as for amplitude modulation), and an accelerating electrodev24 which likewise may-housed for control of beam current. The cathode 21 may be a rectangular body provided with a circular base in which a heater wire is inserted and electrically insulated from the cathode; one end of the heater wire may be connected to the cathode. The constructionand manner of mounting of the electron `gun 20 may be as shown .in a copending application of Roy A. 'Paananem Serial No. 717,897, filed February 27, 1958, now UnitedStates kLetters Patent No. 2,914,700, issued Novemberv 24, 1959,

assigned to the assignee ofthe present application.

Electrical energy 'from appropriate sources issupplied tothe cathode 21, the heater, grid 'electrode 23 and acprovided with a glass seal 69 for sealing the traveling wave tube 10 after evacuation. One end of sole-stipporting member 54 contains an outwardly flared portion which is connected to the inner surface of sleeve 68. The leads 61, 62, 63, and 64 are mounted in electrically `'insulated relation with supporting member 54 by one or more glass beads 71.

The coaxial output coupling means 17 is sealed in an opening of wall 36 of delay lline l2 and is impedancematched to vthe delay line. The inner conductor 73 of the Output coupling means 17 ispconnected to a finger of delay line 12 at or near the end of the delay line adjacent electron gun 20. p

The oscillator 10 may be locked in frequency vby an external source, 'such as shown and described in the aforesaid copending application of Dench and La Rue, Serial No. 562,472, now United States Letters Patent No. 2,888,649, issued May 26, 1959, in which case an additional energy coupling means would be coupled to the end of the delay line 12 opposite output coupling means 17. p

Traveling Awave tube l10 may be provided with a collec tor electrode 75, shown in Fig. 2, for intercepting electrons after one traversal of the arcuate interaction space 50. This collector electrode may be in the form of a projection from back wall 36 of delay structure'v12. In some instances, however, the collector electrode may be omitted and the electron stream made reentrant. i

The necessary electric field between the slow-wave structure 12 and sole 14 may be obtained by means of a unidirectional voltage applied therebetween; such a voltage may be supplied by a battery 83. 'The sole 414 may be biased negatively relative to the cathode Y21 by means of a source '81 of voltage connected between cathode lead ,61 and sole-supporting member 54.b`y way of tube 68. The cathode 21 may in some instances, however, be at the same potential as sole 14; `in this case, source 81 wouldrbe omitted. Similarly, the delay line 12 may be maintained at a potential positive relative to both sole 14 and cathode 21 by means of the source 83 of unidirectional voltage connected between the cathode and sleeve 66, the latter being connected,`in turn, to delay line 1.2. The accelerating electrode 24 may be maintained and cathode lead 61.

A uniform magnetic field transverse to the direction of propagation of the electron beam is provided by a permanent magnet or electromagnet having cylindrical pole pieces 91 and 92 radially positioned on or adjacent the anode cover plates 38 and 39, respectively. The gap between pole pieces 91 and 92 is indicated as M in Fig. 1. Pole piece 91 is apertured to receive lead-in assembly 15, while pole piece 92 is apertured to maintain symmetry of the magnetic field. The flux lines should be concentrated in the interaction space 50 between sole 14 and delay line 12. By proper adjustment of the magnitude and polarity of the magnetic and electric fields so established, the electron beam may be made to show a more or less circular path along the interaction space 50 under the combined inliuence of these transversely disposed fields.

This invention is not limited to the particular details` of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A slow wave energy propagating structure comprising a basemember and a plurality of periodically spaced elements each including three successively arranged mutually perpendicular coplanar portions, the first of said portions being disposed substantially perpendicular to said base member and having its free termination attached to said base member, said second portion being disposed substantially parallel to said base member, and said third portion extending substantially perpendicular to said base member and having the free termination thereof spaced from a reference surface disposed substantially parallel to said record portion and passing through the intersection of said first portion and said base member, said free termination being disposed on the same side of said reference surface as said second portion, saidk elements being of uniform cross section throughout the length thereof.

2. A slow wave energy propagating structure comprising a base member and a plurality of periodically spaced elements each including three successively arranged mutually perpendicular coplanar portions, the first of said portions being disposed substantially perpendicular to said base member and having its free termination attached to said base member, said second portion being disposed substantially parallel to said base member, and said third portion extending substantially perpendicular to said base member and having the free termination thereof spaced from a reference surface disposed substantially parallel to said second portion and passing through the intersection of said first portion and said base member, said spacing being substantially equal to the spacing between adjacent elements, said free termination being disposed on the same side of said reference surface as said second portion.

3. A traveling wave electron discharge device comprising a slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated therebetween, means for directing electrons in a beam along said region in energyexchanging relation with said fields of wave energy, said structure comprising a base member and a plurality of periodically spaced elements each including three successivelylarrangedmutually perpendicular coplanar por- `"-tionsgbthefirst-of saidportions being disposed substantially perpendicular Ito said basemember and having its free termination YNattached to "said base member, said second portion `beilztgdisposed substantially parallel to said base member, andsaidwthird portion extending substantially -r perpendicular to saidrbase member `and having the free `termination thereof spaced from a reference surface disposed substantially parallel `to said second portion `and passing through the intersection of said first portion and said` base member, t said `spacing being substantially equal to the spacing between adjacent elements, said free termination being disposed on the same side of said reference surface as said second portion.

` 4. A traveling wave electron discharge device comprising a slow wave energy propagating structure producing in the region adjacent thereto fields of electro-v the free termination thereof spaced from a reference surface disposed substantially parallel to said second portion and passing through the intersection of said first portion and said, base member, said free termination being disposed `on the same side of said reference surface as said second portion.

5. A traveling wave electron discharge device comprising a slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy being propagated therebetween, means for directing electrons in a beam along said region in energy-exchanging relation with said fields of wave energy, said structure comprising a base member and a plurality of periodically spaced elements each including three successively arranged mutually perpendicular coplanar portions, the first of said portions being disposed substantially perpendicular to said base member and having its free termination attached to said base member, said second portion b'eing disposed substantially parallel to said base member, said third portion extending substantially perpendicular to said base member and having the free termination thereof spaced from a reference surface disposed substantially parallel to said second portion and passing `through the intersection of said first portion and said base member, said spacing being substantially equal to the spacing between adjacent elements, the length of said second portion being substantially less than that of said first portion, said free termination being disposed on the same side of said reference surface as said second portion.

6. A slow wave energy propagating structure comprist ing abase member and a plurality of periodically spaced elements each including three successively arranged mutually perpendicular coplanar portions, the first of said portions being disposed substantially perpendicular to said `base member and having its free termination attached :T7 v '8 elements each incldingnthree suocessively -.arrange d ymu fspacing between yadjacent elements, said free termination tually Aperpendicular coplanarvportions, the first tof .said :being disposed on the same 'side -of said reference surface .portions :being '-,disposed-substantially perpendicular to as said fsecond ,portion said baseimemberiandihaving' its ireetermination attached t A i A to said base member, Iand said third portion 'extending 5 -Refel'ellescifed ln the me 0f hlS Dfi'llt substantially perpendicular to said .hase ymemberandhavl l UNITED .STATES PATENTS 111g the free termmationthereof .spaced ironia reference 1 y n surface disposedsubstantiallygparallel `to a second portion 2,827,588 Guenafd et a1- Man 18 ,1958 and passing through the intersection of said rstgportion rr y s t and the -portion -of vsaid lbase member vfacing -fsaid lfree 10 FOREIGN PATENTS termination, said fspacingabeng substantially equal-to the 699,890 Great Britain iNov. 18, 1953 

